1. Field of the Invention
This invention relates to high speed impact printers of the type generally employed in data processing systems. Typically, such systems include a hammer bank assembly having a plurality of flat bodied print hammers arranged in a row and interleaved with a plurality of magnets. In particular, the invention is directed to a printer assembly of the type which utilizes a moving band which contains a raised type font. The band moves along an impact line and when a particular character is positioned in a desired print position, the hammer for that position is energized to impact against the band so as to cause the character to be printed.
2. Prior Art
Examples of hammer bank assemblies useable in band printers are shown in U.S. Pat. Nos. 3,643,595 to Helms and 3,983,806 to Ishi. Both of these patents disclose a hammer bank assembly having a plurality of print hammers comprised of a flat coil body section carried by a pair of crossed spring wires and including an impact tip. The hammers are arranged in a spaced relationship along the length of a frame, and are interleaved with a plurality of magnets which are also attached to the frame.
In typical prior art band printer assemblies, the moving character band is formed into a loop and carried by a pair of pulleys. The loop is spaced in front of the hammer bank assembly and a ribbon is passed between the moving character band and the hammer bank. A platen is located within the character band loop. A print medium such as paper is passed between the hammer bank and the ribbon and the hammers are impacted against the platen in a controlled fashion in order to accomplish printing. An example of such a prior art configuration is shown in FIG. 1.
The prior art configuration described above has several disadvantages associated with it. Initially, the hammer bank and character band must be accurately positioned with respect to one another. In order to accomplish this, the two components are generally mounted upon a cast main frame which must be very accurately machined. These main frames are also generally used to support a ribbon cassette assembly and a paper tractor drive assembly. The frame makes up a significant portion of the cost of mechanical items in a printer. In addition, various frames must be separately designed and fabricated for use with different sized hammer bank assemblies. This increases the cost of manufacture of a family of printer assemblies.
In addition to the cost and manufacturing difficulty involved when using a cast and machined main frame, the prior art configuration presents significant accessibility problems. Because the hammer bank assembly is located on one side of the print medium and the character band assembly on the other, access to the assembly for loading and unloading of the print medium is severely restricted. Paper loading is generally done from below the assembly near the middle of the device. Because of the relative inaccessibility to the paper feed path, the hammer bank assembly is typically mounted upon a movable subframe which is mounted upon the main fame. The subframe is moved away from the character band during loading of paper into the printer. This further increases the complexity and cost of the assembly.
Several prior art printer assemblies position the hammer bank assembly within the loop formed by the moving character band. Assemblies of this type are shown in U.S. Pat. Nos. 3,585,927 to Burns et al., 3,719,139 to Niccolai and 3,741,110 to Bossi. In all of these printers, the type font is located on the outside of the moving band and the hammers are impacted to force the type characters against a platen located behind the hammer bank and character band assemblies. Although these printers may provide some reduction in the size of the assembly, they provide little or no improvement in ease of loading or visibility of the print medium.